1. Field of the Invention
The present invention relates to a process for preparing block copolymers by anionic polymerization in tubular reactors.
The preparation of block copolymers from anionically polymerizable monomers has been known for a long time, and is generally performed in a batchwise manner in a stirred vessel, as described for example in German Patents 1,301,496 and 2,550,227.
In order to obtain polymer blocks having sharp transitions, the individual monomers must be reacted strictly sequentially, ie. in each case all of a monomer must have reacted before the next monomer is added. In principle this also applies to tapered transitions if they are to remain controlled, ie. if after the transition a purely homopolymeric block is finally to be obtained again. Only polymer solutions of low concentration are obtained, since otherwise the temperature increases to such an extent due to the polymerization that a thermal termination of the polymer chain occurs. All this results in a very poor space-time yield.
2. Description of Related Art
The preparation of block copolymers in a reactor cascade is described in DE-A-2111 966. The space-time yield may indeed be more favorable in this process if complete polymerization in the individual reaction zones is dispensed with, but then only polymers having regions with a random or largely random monomer distribution, ie. tapered transitions, are obtained.